Don't quote me on this but I think its okay as long as the fertilizer has set in for a few days or it has rained since putting it down. Similair to how there are "keep pets off" signs that you put up right after you put it down.

Be aware that there are bugs in the grass and your snake could get a bug on it. You probably won't get snake mites or anything, but just something to consider. I would try and keep the snake out of direct sunlight as well. In the sun temps can spike pretty high.